Two items:
* Check with Tesla, but I'd assume the 40kWh virtual limit is distributed across all cells therefore charging to your 100% actually means charging only to part of the capacity and hence no degradation.
* You don't need twin chargers for supercharging. You only need to enable supercharging in software (i.e. fork over the fee). Superchargers bypass the onboard chargers as they have them inside the SC facility. Also, twin chargers give you 22kW, superchargers go at 90-120kW
* Twin chargers are useful if there are locations where you can use them. If you install a HPWC at home and actually have the infrastructure to feed it at high amps too, then it'll for sure cut the time in half allowing for faster turn around if that's what you need. For me it was a no brainer as in Estonia all chargers are 400V 32A 3-phase i.e. 22kW and require twin chargers to take advantage of, but at home I'm gonna charge at or below what one charger can do as at home I don't have as many amps available.